In the printing world, artwork is typically first plotted onto a film in an imagesetter or plotter. There are many types of imagesetters, three of the common ones are flat bed, external drum and internal drum imagesetters. A flat bed imagesetter scans film which has been placed onto a flat surface. External and internal drum imagesetters scan film which has been placed on the external and internal surface, respectively, of a cylindrical drum.
After plotting in an imagesetter, the artwork is transferred to a printing plate typically via a contact process. The printing plate is later to be placed on a printing press which prints the artwork onto pieces of paper or other permanent medium.
The contact process typically involves illuminating the printing plate which is placed the film, with a light bulb. New types of printing plates, such as the N90 manufactured by Hoechst AG of Germany, have recently been developed. These printing plates are typically formed of a metal or polyester sheet onto which is placed a light sensitive emulsion which is sensitive to scanned laser light. The artwork can be directly scanned onto these new printing plates, thereby potentially eliminating the need for the step of plotting on film.
However, the plotters which plot the artwork onto films cannot directly be utilized for plotting on the new printing plates for two main reasons: 1) the printing plates are less sensitive to laser light than film is and 2) the metal printing plates are not as flexible as the film.
The issue of sensitivity to laser light is typically solved by utilizing a stronger laser source. This entails also changing the associated optics to those which perform the same function but on the stronger laser beam.
The issue of rigidity is more difficult to solve. For a flat bed plotter, such as the Raystar CTP manufactured by Scitex Corporation Ltd. the assignees of the present invention, there is no problem since the printing plate is laid flat. However, flat bed plotters, for both films and printing plates, utilize a tremendous amount of floor space. External and internal drum plotters utilize less floor space and internal drum plotters provide the fastest scanning speed. Thus, an internal drum printing plate plotter has many advantages.
Reference is now made to FIG. 1 which illustrates a cross-section of a typical internal drum plotter for film. The unexposed film 10 is stored in an input cassette 12 and the exposed film 10' is stored in an output or takeup cassette 14. Between the two is the drum 16 against whose inner surface the film 10" to be exposed is laid, sometimes with the help of a vacuum system (not shown).
As shown, the film 10 follows a very curved path which is convex when entering and exiting the drum 16 (see reference numerals 18) and concave within the drum 16. Such a path is not possible for a printing plate; due to the rigidity of the plate, it cannot make such tight turns without becoming creased.